1. Field of the Invention
The present invention relates to a zoom lens and an imaging apparatus. In particular, the present invention relates to a zoom lens appropriate for use in a digital camera, a digital video camera, and the like that use imaging devices, such as a CCD (Charge Coupled Device) and a CMOS (Complementary Metal Oxide Semiconductor). Further, the present invention relates to an imaging apparatus including the zoom lens.
2. Description of the Related Art
Conventionally, zoom lenses were used in various cameras. Especially, three-group zoom lenses were widely used, because they are compact and can correct aberrations in an excellent manner. Meanwhile, digital cameras and digital video cameras became rapidly popular in recent years. In the digital cameras and the digital video cameras, it is desirable to reduce the sizes of lenses, and to obtain high quality images with low distortion in a manner similar to lenses of general cameras. Moreover, in the digital cameras and the digital video cameras, it is desirable to reduce the cost for producing lenses. An automatic focusing technique is mainly used in the digital cameras and the digital video cameras, and high-speed focusing is desirable. Therefore, as a method for focusing a zoom lens, an inner focus method and a rear focus method are often used. The inner focus method and the rear focus method can reduce the weight of the lens, and the lens is easily drivable, because the lens is located close to the body of a camera. In such focusing methods, three-group lenses are more appropriate than two-group lenses. Conventionally, three-group zoom lenses that are small and have zoom ratios of approximately three times were disclosed, for example, in Japanese Unexamined Patent Publication No. 2005-321744 (Patent Document 1), Japanese Unexamined Patent Publication No. 2005-331641 (Patent Document 2), Japanese Unexamined Patent Publication No. 2006-139164 (Patent Document 3), U.S. Pat. No. 7,492,527 (Patent Document 4), Japanese Unexamined Patent Publication No. 2008-233499 (Patent Document 5), and Japanese Unexamined Patent Publication No. 2000-267009 (Patent Document 6).
However, in the zoom lenses of the aforementioned fields, a demand for improving the performance of the zoom lens, a demand for reducing the size of the zoom lens, and a demand for lowering the cost for producing the zoom lens became tougher every year. The zoom lenses disclosed in Patent Documents 1 and 2 have compact structure, in which the size of each lens group in the direction of the optical axis is reduced. However, since Patent Documents 1 and 2 use glass as the material of aspheric lenses, the cost for producing the zoom lens is high, and it is difficult to lower the cost to a desirable level of recent years. The zoom lenses disclosed in Patent Documents 3 through 6 use many plastic lenses. Therefore, they can reduce the cost for producing the lenses and the weight of the lenses. However, the sizes of the zoom lenses of Patent Documents 3 through 6 need to be further reduced to satisfy a demand for small zoom lenses of recent years.